An ink jet printer is an example of a printing apparatus that ejects droplets of ink onto a recording medium such as a sheet of paper, for printing an image on the recording medium. The ink jet printer includes a head unit having at least one ink jet head provided with an ink cartridge that accommodates the ink. In operation of the head unit, the ink is supplied from the ink cartridge to each ink jet head having ejection nozzles, so that a printing operation is performed by ejection of the ink droplets from selected ejection nozzles.
However, ink jet printers may suffer from a problem of evaporation of solvent from the ink causing an increase in the ink viscosity that leads to nozzle clogging and the inability to fire an ink droplet under normal conditions. A clogged nozzle may not only result in diminished print quality, but may require the expense of replacing the entire print head. To solve this problem, there has been practiced a so-called “flushing operation” wherein the ink is forcibly discharged from the ejection nozzles which are open in a nozzle surface of each ink jet head.
Several flushing methods in existence have undesirable effects upon image quality. One such flushing method involves printing a line across the top or bottom of each page to flush the nozzles. In this method, each nozzle produces multiple drops forming a wide line across the top or bottom of the printed page. The drawback of this approach is that it leaves a large colored line at the bottom of every page and many customers do not have the post-processing equipment to remove it.
Another method involves randomly firing drops from all nozzles at a specified frequency throughout the printing of a job. The drawback to this approach is that the indiscriminate firing of the nozzles during the printing can cause excessive background noise and alter the color and accuracy of the printed images.
Intelligent flushing methods exist that lessen the problem of the flushes interfering with image quality, but these methods are also undesirable because they are computationally intensive. Thus, the throughput speed of the printers is negatively affected. One such method involves flushing the color ink dispensing nozzles onto points of the page where black ink will ultimately be printed. By effectively hiding the color ink droplets under black ink from the job data, the image quality may be preserved. However, this method requires additional data processing, slowing down the high speed printing process.
Additional problems exist with the current flushing methods. For instance, current flushing methods require all nozzles to be flushed at the same frequency. This results in wasted ink where a user knows that one color needs to be flushed less frequently than another.
Consequently, what is a needed is a mechanism for flushing the nozzles of an inkjet print head during printing that preserves the integrity of the printed images.